The present invention relates to an optical recording apparatus, and more particularly to an apparatus for tracking information in an optical disc memory which records and plays back the information in real time.
Heretofore, the technique of tracking an information track at an exactitude on the order of .+-.0.1 .mu.m has been indispensable in case of playing back information from the surface of a disc, such as optical video disc and PCM optical disc, on which the information are recorded along tracks in advance. As a typical method therefor, there has been known one in which, as illustrated in FIGS. 1(A) and 1(B), three light spots are formed on the disc surface and are used for the tracking. This method will now be described. In the arrangement of FIG. 1(A), a laser beam 2 emergent from a laser source 1 is collimated by a lens 3 into a parallel beam, which is split into the three light rays of a 0-order diffracted light ray and .+-.1-order diffracted light rays by a diffraction grating 4. Thereafter, the light rays are reflected by a mirror 15 and are condensed as three light spots 6a, 6b and 6c on the disc 7 by a condensing lens 5. The three light spots 6a, 6b and 6c are formed on the track 16 of the disc 7 in a manner to have an inclination .theta. with respect to the direction of the track as shown in FIG. 1(B). A reflected beam from the surface of the disc 7 is led onto photodetectors 10a, 10b and 10c by a semitransparent mirror 8 and a lens 9. At this time, the light spots 6a, 6b and 6c are respectively detected in correspondence with the photodetectors 10a, 10b and 10c. An output from the photodetector 10b is led to a signal playback circuit (not shown) via an amplifier 12. Outputs from the photodetectors 10a and 10c pass through a differential amplifier 11 to become a differential output. After passing through a servo circuit 3, the differential output is led to a driver 14 for the mirror 15 so as to rotate the mirror 15 in correspondence with a tracking error as indicated by arrows, whereby the tracking is performed. More specifically, when the track 16 has deviated rightward as viewed in FIG. 1(A), the quantity of reflected light from the light spot 6a decreases, whereas that from the light spot 6c increases. When the track 16 has deviated leftward, the converse is true. The mirror 15 may be so controlled that the quantities of reflected light from the light spots 6a and 6b, namely, the outputs from the photodetectors 10a and 10c become equal at all times. This method can perform a stable tracking in a case where the information are already formed on the track as in the optical video disc or optical PCM disc. However, it has hitherto had a serious disadvantage as to a digital optical disc which records and plays back digital information, video information etc. in real time.
Hereunder, this disadvantage will be explained. As the digital optical disc, a disc of a structure as shown in FIG. 2(A) (partial enlarged view) has heretofore been used. Referring to the figure, guide grooves 23 of a concave sectional structure having a certain degree of width and depth are previously formed on a substrate 21 of glass or plastics by the use of an ultraviolet-hardenable resin or the like, and a metal film 20 is evaporated thereon. In recording information, a light spot is guided along the guide groove 23 in a manner to track this guide groove, so as to melt the metal film 20 by the light spot. The information are recorded by the melted parts 22 (called "pits"). When the guide groove 23 bearing the information thus recorded is tracked by the arrangement of the light spots as shown in FIG. 1(B), the information are played back by the light spot 6b and the tracking signal is produced by the light spots 6a and 6c, as illustrated in FIG. 2(B). However, the light spot 6a lies in an area where the information is not recorded, and the light spot 6c lies in an area where the information is recorded, so that the quantities of reflected light from the corresponding areas do not equalize even in the absence of the tracking error. Moreover, the inequality varies greatly depending upon the pattern of the recorded information, namely, the arrayal pattern of the pits, so that the precision of the tracking worsens drastically.